Run series

gulpfile.js

const gulp = require('gulp')
const sass = require('gulp-sass')

const transpileSassToCss = (cb) => {
    return gulp.src(`src/sass/main.scss`)
        .pipe(sass())
        .pipe(gulp.dest(`src/css`))
}

const watchSassFile = () => {
    return gulp.watch(['src/sass/**/*.scss'], transpileSassToCss)
}

const beingWatched = (cb) => {
        console.log(`I am being watched`);
        cb();
}

exports.default = gulp.series(beingWatched, watchSassFile)